Very few articles of commerce and general use do not have a coating of some kind for the enhancement of appearance, scratch and abrasion resistance, moisture proofing, adhesion or release of other materials, or alteration of some other characteristics of the surface of the article. Since the quality and cost of the coating is dependent on the coating weight and uniformity it is desirable that a means for monitoring these characteristics be employed.
The use of radiation to measure thickness of coatings has been taught. U.S. Pat. Nos. 3,019,336 and 3,130,303 disclose that beta rays emanating from a radioactive source such as strontium-90 and directed toward a coating are backscattered in an amount related to the thickness of the coating. Such a technique cannot be used to monitor coating uniformity and coating defects during manufacture of web materials bearing thin coatings. In this technique only the average coating weight can be determined, not the weight at any specific point.
U.S. Pat. Nos. 3,675,015, 3,118,060, 3,930,063, 3,956,630 and 4,250,382 disclose methods and apparatus for continuously monitoring coating weight of a coating on a web or fiber. For these methods, a fluorescing compound, dye or pigment, is added to the coating formulation and the fluorescence of the coating in the visible range of the electromagnetic spectrum is continuously and quantitatively measured while the coating is exposed to ultraviolet radiation. Fluorescence readings taken in the cross direction and machine direction of the web give an indication of the lay of the coating on the web. These techniques, although useful for monitoring coatings on some webs, e.g. paper or fiber, are not satisfactory for monitoring coatings on webs which also fluoresce, such as paper treated with an optical brightener to enhance whiteness. Further, it is stated in U.S. Pat. No. 4,250,382 (col. 2, lines 10-12) that fluorescent dyes have not been found satisfactory for detecting coatings of cured polysiloxane resins.